The present invention relates to analog integrated circuits. In particular, the present invention relates to a variable gain amplifier having a constant common mode DC output voltage.
Variable gain amplifiers are commonly used in analog integrated circuits. For example, variable gain amplifiers are used in communication receivers, such as satellite tuners. A typical variable gain amplifier includes a differential input transconductor which converts a differential input voltage signal to a differential input current. A variable gain stage selectively varies the differential input current based on a differential gain control input signal to produce a differential current output. The differential current output is then mirrored into a pair of load resistors to produce a differential output voltage. The AC signal gain from the differential voltage input to the differential voltage output is adjusted by adjusting the current gain through the gain stage. However, as the AC signal levels in the differential output current are varied, the DC levels in these output currents also vary. This varies the DC current levels provided to the load resistors and results in the common mode DC output voltage adversely varying with gain. Therefore, additional circuitry may be required to either compensate for variances in the common mode DC output voltage or reduce their effect before the outputs of the amplifier are applied to subsequent circuit stages.
For example, the amplifier output can be AC-coupled to the next circuit stage through a pair of AC coupling capacitors. These capacitors can be implemented either on-chip or off-chip. On-chip AC coupling capacitors consume considerable semiconductor die area and are limited to fairly small sizes. Off-chip AC coupling capacitors consume input/output pins on the integrated circuit and increase design complexity. Alternatively, stacked-stages can be used with parallel replicated push or pull currents to preserve a common mode DC voltage. These circuits can also increase die area and complexity and with stacked structures can lead to reduced signal swing as the supply voltage reduces.
Improved variable gain amplifiers are therefore desired that are capable of operating at reduced supply voltages and varying gain while maintaining a substantially constant common mode DC output voltage.
One embodiment of the present invention is directed to a variable gain amplifier, which includes a differential transconductor, a differential gain stage, a DC compensation circuit and first and second load resistors. The differential transconductor has first and second differential voltage signal inputs and first and second differential current outputs. The differential gain stage selectively steers current from the first and second differential current outputs to first and second variable current outputs, respectively, and to first and second compensation current outputs, respectively, based on a differential gain control input. The first and second load resistors are coupled to the first and second variable current outputs, respectively. The DC compensation circuit combines current in the first and second compensation current outputs to form a DC compensation current and couples the DC compensation current to the first and second load resistors.
Another embodiment of the present invention is directed to a differential variable gain amplifier, which includes a differential voltage signal input, a differential gain control input, a differential voltage signal output and first and second load resistors which are coupled to respective terminals of the differential voltage signal output. A conversion circuit converts the differential voltage signal input to a differential input current. A variable gain circuit produces a variable differential output current and a DC compensation current from the differential input current based on the differential gain control input. An output circuit feeds the differential output current through the first and second load resistors. A DC compensation circuit injects the DC compensation current into the first and second load resistors.
Another embodiment of the present invention is directed to a method of varying gain from a differential voltage input signal to a differential voltage output signal based on a differential gain control signal. The method includes: converting the differential voltage input signal to a differential input current having first and second input current legs; selectively steering current away from the first and second input current legs based on the differential gain control signal to produce first and second variable output currents, respectively, and first and second compensation currents, respectively; feeding the first and second variable output currents through the first and second load resistors, respectively; and summing the first and second output currents to produce a DC compensation current and injecting the DC compensation current into the first and second load resistors.